The present invention relates to optical encoders and, more particularly, to encoders which may be used to provide print control information in an ink jet copier for copying all types of materials, including printed text and photographs, and which is particularly useful in multiple color printing operations. In recent years, ink jet printing has developed in sophistication to the point where it is now possible to print both text material and other types of material, such as photographs, with a high degree of resolution. A technique similar to half-tone printing is used to reproduce photographs and other graphics. The photograph is reproduced by depositing ink drops closely together in areas which are to be dark in tone and further apart in areas which are to be lighter in tone. In a color printing arrangement, drops of red, yellow, and blue ink are deposited on the copy paper with the spacing between drops of each color ink being dependent upon the color content of the image in the area being reproduced.
In order to utilize an ink jet printer as the printing mechanism for the copier, it is necessary to have some method of scanning the master to produce print control information which will space the ink drops apart by the necessary distance to reproduce the master. When printing with multiple colors of ink, it is necessary that the master be scanned, generally by using color filters, as to the color content of each color of ink being printed. The final multicolor copy is produced by superimposing the printed images which result from scanning the master with respect to its color content for each color of ink being used.
It is known in the prior art to scan optically a master by illuminating the master and then, by some arrangement, directing the reflected light from along a scan line on the master image to one or more optical lenses which, in turn, direct the light to a photoelectric transducer. The output from the photoelectric transducer provides an indication of the printed material along the scan line of the master, with more light being reflected from the lighter toned areas, indicating little or no ink deposits in these areas.
In order to provide scanning for a particular color, a color filter is inserted in the optical system which passes only light of a preselected frequency range. The color scanning will usually be substracted--i.e., the color filters in the optical scanner block out the color being scanned. If, for instance, red is being scanned in the master and a red portion of the image is encountered, the light reflected from the master image will be of wavelengths principally that are in the red region of the light spectrum, with the balance of light being absorbed by the master. The color filter will block out the red spectrum light, permitting light of other wavelengths to pass. A large amount of light striking a photoelectric transducer in the optical scanning system will, therefore, indicate that a region having a small amount of red color content is being scanned. On the other hand, when very little light is directed to the photoelectric transducer, this will indicate that the red content of the area being scanned is significant. It is usual then to substract the output from the photoelectric transducer from a reference level to produce a signal directly related to the color content in the master image of the color being scanned.
Several problems have been noted with respect to prior art scanning devices. It has been found that the optical path alternation in a scanner will vary over a period of time for one of a number of reasons. If the supply voltage to the lamps which illuminate the master image should fluctuate, the intensity of the light directed to the photoelectric transducer, and therefore the magnitude of the electrical signal produced thereby, will fluctuate. Additionally, when a number of colors are being scanned in the master image by a scanner in which filters of various color transmitting characteristics are sequentially inserted into the optical path, the amount of light falling upon the photoelectric transducer will vary in dependence upon the light transmitting characteristics of each of the filters being used. It is not possible to obtain a set of filters of various colors which are perfectly matched in their light transmitting characteristics.
Another problem which has been noted is the image distortion which results from fluctuations in the speed at which the master image is scanned. If, for instance, the master image is mounted on a rotating drum and the drum rotated past the scanner, it is apparent that fluctuations in drum speed will distort the scanned signal. A straight line which is transverse to the direction of scanning may, as a consequence, be printed as a wavy line if the scanning drum rotation fluctuation is sufficiently severe.
Thus, it is seen that there is a need for a scanner for use with an ink jet printer copier in which variations in the rate at which the master image is scanned are detected and compensated and, further, in which variations in the light transmitting characteristics and transducing characteristics of the optical scanner are compensated.